1. Field of the Invention
This invention relates to a high-speed, high-efficiency, high-precision voltage control system for an ink jet printer.
2. Description of the Prior Art
In thermal ink jet printing, droplets of ink are selectively emitted from a plurality of drop ejectors in a printhead, in accordance with digital instructions, to create a desired image on a surface. The printhead typically comprises a linear array of ejectors for conveying the ink to a sheet. The printhead may move back and forth relative to a surface, for example to print characters, or the linear array may extend across the entire width of a sheet (e.g. a sheet of plain paper) moving relative to the printhead. The ejectors typically comprise capillary channels, or other ink passageways, forming nozzles which are connected to one or more common ink supply manifolds. Ink from the manifold is retained within each channel until, in response to an appropriate digital signal, the ink in the channel is rapidly heated and vaporized by a heating element disposed within the channel. This rapid vaporization of the ink creates a bubble which causes a quantity of ink to be ejected through the nozzle to the sheet.
U.S. Pat. No. 5,223,853 to Wysocki et al., entitled "Electronic Spot Size Control in a Thermal Ink Jet Printer," which is assigned to the assignee of the present application, is herein incorporated by reference. Wysocki et al. discloses a system by which ink jet droplets of a consistent size are ejected from a thermal ink jet printhead, regardless of the original temperature of the liquid ink in the printhead. As described in detail in Wysocki et al., one of the most crucial parameters for image quality in an ink jet printer is the spot size of individual droplets of ink emitted from the printhead, and in turn an important parameter for determining the spot size of individual droplets is the temperature of the liquid ink immediately before ejection. The system of Wysocki et al. operates on the principle of first measuring the temperature of the liquid ink in the printhead and then, in response to this measured temperature, providing to the printhead an optimal combination of power (typically voltage) and pulse duration to a heating element which is used to vaporize the liquid ink and cause it to be ejected from the printhead. From the standpoints of preventing overheating of the printhead chip, avoiding kogation of ink within the printhead, and other practical concerns, a selected pulse duration must be coupled with an appropriate power level, and vice-versa. For each measured temperature there is provided in this system a best combination of amplitude and pulse width, as opposed to a system which merely increases or decreases one input or the other.
In a typical configuration of a thermal ink jet printhead, a linear array of ejectors, in one embodiment, has 128 ejectors spaced 300 to the inch, which is caused to move across a sheet on which an image is to be printed. The ejectors are activated, as necessary according to the desired image, in groups of four ejectors at a time across the array, while the array itself may move across the sheet. Thus, in some situations a set of ejectors may be activated every 4 microseconds. In order to realize a spot size control system such as that disclosed in Wysocki et al., a system for providing the best combination of power and pulse duration must be able to react to very quick changes in temperature of the liquid ink in the printhead, particularly as the action of the printhead itself is the main contributor to changes in temperature of the liquid ink. In a realistic practical application, the temperature of liquid ink in the printhead could change significantly within 200 microseconds. Further, because the necessary combination of voltage and pulse duration for one temperature may be completely different from that for the "neighboring" temperature, a control system may have to change its voltage very abruptly, for example, from 38 volts to 41 volts, in a stepwise manner with minimal time for transition.
In addition to the problems of time lag associated with traditional analog voltage-supply devices such as that used in Wysocki et al., which can typically change voltage at no more that 1 volt per millisecond, there is the problem that such power supply devices operate on the principle of "throwing voltage away" when a lesser voltage is suddenly needed. When voltage is reduced by an analog circuit, the power which is not output in the form of a voltage is at least temporarily dissipated as heat. When such heat is generated in components such as partially turned-on power transistors, the component performance can degrade, so that expensive cooling means are required to effectively remove the heat. For these reasons, a power supply system is needed for ink jet printers which is of very quick response, and which provides accurate voltage levels as needed without generation of excess heat in components whose performance or reliability is very dependent upon temperature.
In the prior art, U.S. Pat. No. 5,017,948 discloses a control system for an ink-jet printer in which the voltage applied to the heating elements is adjusted as a function of the number of heating elements actuated at a given time.
U.S. Pat. No. 5,083,137 discloses a control system for an ink-jet printer in which the voltage applied to the heating elements is compared to a reference voltage through a comparator, the output of the comparator being applied to the gate of a PMOS driver driving the heating elements.